A number of mutually corroborative physical and chemical techniques will be used to locate the binding sites of actin of the following species: a) Tropomyosin, b) the troponin-tropomysin complex in the absence or presence of calcium, c) Myosin in the absence or presence of ATP. The proximity relationships between these binding sites will have bearing on whether thin filament regulation takes place by a steric blocking type of mechanism or a conformational change type of mechanism. These techniques include fluorescence and triplet probe methods, electron microscopy, chemical cross-linking and surface labeling.